Process for manufacturing a solar cell foil using a temporary substrate

ABSTRACT

The invention pertains to a process for manufacturing a solar cell foil comprising the steps of: providing an etchable temporary substrate applying a front electrode of a transparent conductive oxide (TCO) onto the temporary substrate applying a photovoltaic layer onto the TCO layer applying a back electrode layer applying a permanent carrier ensuring that the front electrode and the back electrode are electrically connected in an interconnect to establish a series connection, the front and the back electrode each being interrupted by front and back groove, respectively, at different sides of the interconnect in any one of the preceding steps providing an etch resist on the non-TCO side of the temporary substrate at least at the location of the interconnect, and at least not at the entire location of the front groove selectively removing the temporary substrate where it is not covered with etch resist. The process of the invention provides a cap of a protective material on the interconnect, leading to a solar cell foil with improved properties.

The invention pertains to a process for manufacturing a solar cell unitusing a temporary substrate. The invention also pertains to the solarcell foils thus obtained.

Solar cell foils, also known as photovoltaic foils, generally comprise acarrier and a photovoltaic (PV) layer composed of a semiconductormaterial provided between a front electrode comprising a transparentconductive oxide (TCO) (at the front of the foil) and a back electrode(at the back of the foil). The front electrode is transparent, enablingincident light to reach the semiconductor material, where the incidentradiation is converted into electric energy. In this way light can beused to generate electric current, which offers an interestingalternative to, say, fossil fuels or nuclear power.

WO 98/13882 and WO99/49483 describe a method for manufacturing aphotovoltaic foil comprising the steps of providing a temporarysubstrate, applying the transparent conductive oxide, applying aphotovoltaic layer, applying the back electrode layer, applying thecarrier, removing the temporary substrate, and, preferably, applying atransparent protective top coat on the transparent conductor layer. Thismethod enables the roll-to-roll manufacture of a photovoltaic foil ordevice, while at the same time making it possible to use any desiredtransparent conductor material and deposition process withoutjeopardizing the current-generating action of the PV layer. In WO98/13882 an etch resistant may be used to cover the non-adjacent mostleft and most right transparent conductor tracks to provide electriccontact between these side-tracks and the side electrodes, By allowingin such manner part of the temporary substrate to remain in place thesecontacts do not need to be applied separately. WO 01/78156 and WO01/47020 describe variations on this process.

The maximum voltage of a solar cell is determined by the intensity ofthe incident light and by the composition of the cell, moreparticularly, by the nature of the semiconductor material. When thesurface area of the solar cell is increased, more power is generated,but the voltage remains the same. In order to increase the voltage, asolar cell foil is often divided into different cells, which areconnected in series. This is done by providing grooves in the solar cellelectrode layers, e.g., by using a laser or by means of etching or anyother patterning method, and establishing a conductive contact betweenthe front electrode of one cell and the back electrode of another, whileinterrupting the front electrode and the back electrode each at adifferent side of the interconnect so as to prevent short-circuiting.When a solar cell foil is employed, the individual cells are heldtogether by the carrier. The above-mentioned patent applicationsdescribe various ways of effecting the series connection.

It has appeared, however, that the photovoltaic foils prepared by theprocesses described in the above references may suffer from loss ofquality when they are provided with a series connection. It was found tobe difficult to provide photovoltaic foil with a homogeneous and highproduct quality. More in particular, it was found that during theremoval of the temporary substrate by etching, the TCO layer may bedamaged at the location of the interconnect. During the etching step,the etchant may permeate the damaged TCO layer and enter into theinterconnect, therewith disrupting it. One of the problems is thepossibility of occurrence of short-circuit between two separated partsof TCO layer by remaining temporary substrate. There is therefore a needfor a more robust process for preparing a solar cell foil provided witha series connection via a temporary substrate, one which results in amore homogeneous product of higher quality and minimal short-circuitproblems. The present invention provides such a process.

The present invention thus pertains to a process for manufacturing asolar cell foil comprising the steps of:

-   -   providing an etchable temporary substrate    -   applying a front electrode of a transparent conductive oxide        (TCO) onto the temporary substrate    -   applying a photovoltaic layer onto the TCO layer    -   applying a back electrode layer    -   applying a permanent carrier    -   ensuring that the front electrode and the back electrode are        electrically connected in an interconnect to establish a series        connection, the front and the back electrode each being        interrupted by front and back groove, respectively, at different        sides of the interconnect    -   in any one of the preceding steps providing an etch resist on        the non-TCO side of the temporary substrate at least at the        location of the interconnect, and at least not at the entire        location of the front groove    -   selectively removing the temporary substrate where it is not        covered with etch resist.

The result of the process according to the invention is that at thelocation of the interconnect parts of the temporary substrate remain onthe TCO layer. The presence of the resulting protective caps oftemporary substrate material has been found to improve the properties ofthe solar cell foil as regards homogeneity and quality. More inparticular, solar cell foils prepared by the process according to theinvention show fewer non-functioning cells than comparable solar cellfoils where the temporary substrate is removed in its entirety.

The present invention also pertains to a solar cell unit comprising afront electrode, a PV layer, and a back electrode layer, wherein thesolar cell unit is divided into at least two individual cells connectedin series, the series connection comprising an interconnect whichelectrically connects the front electrode of one cell with the backelectrode of an adjacent cell, while the front and the back electrodeare each interrupted at different sides of the interconnect, in whichsolar cell unit a protective cap is present on the front electrode atthe location of the interconnect, with the protective cap being of adifferent material than the interconnect. This solar cell unit can beobtained through the process of the present idea, but also throughdifferent processes. Preferably, the solar cell unit is a flexible solarcell foil suitable for handling in a roll-to roll process.

The process and the solar cell unit of the present invention haveseveral additional advantages.

In the first place, if the protective cap is made up of a conductivematerial, e.g., when it originates from a temporary substrate made of aconductive material, the protective cap can function as part of theinterconnect, thus improving the conductive properties of theinterconnect. This also allows for less stringent control of theconditions applied in manufacturing the interconnect as regards theproperties of the TCO, because even if the TCO is damaged during themanufacture of the interconnect, the presence of the protective cap ofconductive temporary substrate material ensures that the interconnectwill function properly.

Further, by appropriate connection, the strip of temporary substrateremaining on the TCO layer can be made to function as part of a currentcollection grid, that is, a pattern of lines of a conductive materialwhich is applied in such a way as to enable easy collection of thecurrent carriers (electrons) generated in the photovoltaic layer andflowing through the electrode. As the solar cell does not generateenergy at the location of the interconnect, the interconnect is an ideallocation for the grid, since its presence at that location does notdetrimentally affect the current-generating properties of the solar cellfoil.

Further, the presence of conductive strips on the interconnect makes iteasier to divide the solar cell foil into individual units, since theconductive strips can be formatted in such a manner that they can beused for attaching electrodes to connect the solar cell foil to the“outside world”.

In the context of the present specification, the term etching isintended to mean removing by chemical means, e.g., dissolution. Anetchable substrate is a substrate which can be removed by chemicalmeans; an etch resist is a material which can resist at leasttemporarily the conditions applied during the removal of the temporarysubstrate. The etch resist can be any material which can be applied tothe temporary substrate at the location of the interconnect and whichwill protect the temporary substrate at least temporarily from theaction of the etchant. Various types of etch resist may be envisaged. Inthe first place, there is the etch resist which is resistant to theactions of the etchant and which will not be removed from the temporarysubstrate at some stage of the process. This type of etch resist isindicated as a permanent etch resist. In contrast, one may also envisagea temporary etch resist, viz. an etch resist that will be removed atsome further stage of the process.

The formation of protective caps from the temporary substrate using theetch resist may be effected in various ways. For example, one may applyan etch resist, carry out the etching step, and optionally remove theetch resist in some further step. It may be that the temporary substrateis thicker than is desired for the protective cap. In that case, one canfirst etch part of the temporary substrate, then apply the etch resist,and subsequently remove the unprotected portion of the temporarysubstrate. In that case one may also first apply a temporary etchresist, followed by partial removal of the temporary substrate where itis not protected by an etch resist. Then, the temporary etch resist isremoved and a further etching step is performed to reduce the thicknessof the protective cap. To effect the latter option it is also possibleto use a material which is only temporarily resistant to the etchant. Ifthis material, which is in effect an etch retardant, is used, thefollowing will take place. At first, the etch retardant ensures thatetching will take place only at the location where the temporarysubstrate is not protected by the etch retardant. Then, when the etchretardant has dissolved, the etchant ensures that the thickness of theprotective cap is reduced.

A specific embodiment of the process of the invention is one in whichthe etch resist is a permanent etch resist the color of which has beenselected such that the caps on the interconnect have a color whichmatches that of the energy-generating part of the solar cell unit orcontrasts with it.

The color difference between the energy-generating part of the solarcell unit and a colored interconnect cap can be expressed by way of thedEab, which is defined as follows:dEab=(dL ² +da ² +db ²)^(1/2)

wherein dL, da, and db are the differences in brightness, blueness, andredness, respectively, between the parts provided with coloring materialand the energy-generating parts of the solar cell unit. The L, a, and bvalues can be determined in accordance with the CIELAB procedure using aD65 light source. If the color of the interconnect caps is to match thatof the solar cell unit, the dEab generally is below about 5, preferablybelow about 2, more preferably, below about 0.3. In that case one canspeak of the use of a camouflage color. If the color of the interconnectcap is selected to contrast with that of the energy-generating part ofthe solar cell unit, the dEab value generally is above about 10,preferably above about 12, more preferably between about 20 and 100. Ifmore than one color is used, generally at least one of these colors willsatisfy the above requirements for the dEab value.

The use of a combination of a distinguishing color and a matching colormakes it possible to decorate the solar cell unit with colored designson a homogeneous background. Examples of envisaged designs are patterns,letters, figures, stripes, rectangles, and squares. In this embodiment,generally 10-90% of the etch resist is provided with a distinguishingcolor, while 90-10% of the etch resist is provided with a camouflagecolor.

The application of the etch resist onto the temporary substrate can becarried out at any stage in the process according to the invention. Itcan, e.g., be applied before the beginning of the process, that is,before the application of the TCO onto the other side of the temporarysubstrate. It can be applied at any intermediate stage, and it can beapplied at the end of the process, that is, after the application of theback electrode or, where applicable, the permanent carrier, and justbefore removal of the temporary substrate by etching. The latter optionis preferred, because it prevents the etch resist pattern from beingdamaged during the preceding parts of the process. It also prevents thepresence of the etch resist pattern on the “back” of the temporarysubstrate from interfering with the other processing steps. In thepreferred roll-to-roll embodiment of the process according to theinvention both may happen if the temporary substrate provided with apattern in an etch resist on the back is led over one or more rolls. Theapplication of the etch resist before the application of the transparentconductive oxide layer has the additional disadvantage that the etchresist should be able to withstand the high temperatures which are oftenapplied in the deposition of the transparent conductive oxide layer.

In a preferred embodiment of the process according to the invention, thetemporary substrate is flexible, a flexible permanent carrier isapplied, and the process is carried out by way of a roll-to-rollprocess.

The Series Connection

The manner in which the series connection is carried out is not criticalto the present invention. In general, the series connection isestablished by ensuring that the front electrode of one cell and theback electrode of an adjacent cell are electrically connected in aninterconnect while the front and the back electrode are each interruptedat different sides of the interconnect.

As indicated above, in each solar cell unit, the back electrode and thefront electrode (TCO layer) are interrupted, each at the other side ofthe interconnect. For both interruptions, the interrupting groove ispresent through the electrode layer itself and optionally also throughthe PV layer. For some of the embodiments to be discussed below, thepresence of the groove through the PV layer will be describedexplicitly, but it may be applied in general. It should be noted,however, that if the interrupting grooves are present through theelectrode layer and through the PV layer, care should be take to ensurethat the electrode is not inadvertently short-circuited with the otherelectrode through the groove in the PV layer. Strips of an insulatingmaterial may be applied at the location where the above-mentionedinterrupting grooves are to be applied, e.g., by laser-scribing. Theinsulating material will act as laser stop to prevent the formation ofshort-circuits by melting of the underlying material. For some of theembodiments to be discussed below, the provision of strips of aninsulating material will be described explicitly, but they may beapplied also in other embodiments. The insulating material may beapplied on the TCO layer and/or on the PV layer, as applicable.

The interconnect can be established in various manners. It can beestablished by applying a groove through the PV layer and filling itwith a conductive material. The groove may or may not extend through theTCO layer, through the back electrode layer, or through both the TCOlayer and the back electrode layer. This will depend, int. al., on theprocess step in which the groove is applied. In one embodiment, theconductive material which is used to fill the groove in the PV layer isback electrode material. If the PV layer is an amorphous Si layer, theconductive connection can also be established by fusing the amorphous Silayer with the back electrode. If the PV layer is an amorphous Si layer,the interconnect can also be established by recrystallising the Si intoa conductive material. This option is less preferred, however.

The sequence in which the interconnect and the interrupting grooves areapplied is not critical and will depend on what is most suitable in thespecific process applied.

The width of the interconnect and the grooves interrupting the TCO layerand the back electrode layer generally is determined by the followingconsiderations. At the sites of the interconnect and the interruptinggrooves, the solar cell is unable to convert light into electricity. Inview of this, the interconnect and the grooves have to be as narrow aspossible. On the other hand, they have to be wide enough to ensure thatthe desired effect, the interruption of the different layers and theconnection of the front and the back electrode with sufficientconductivity, is attained. The grooves interrupting the TCO layer andthe back electrode layer generally have a width of 2 to 200 μm,preferably of 5 to 75 μm. The interconnect generally has a width of 20to 200 μm, preferably of 50 to 150 μm.

The strips of an insulating material that may be applied at the locationwhere the above-mentioned interrupting grooves are to be applied shouldbe wide enough to allow for easy provision of the interrupting grooveswithin the width of the strips. In general, the strips have a widthbetween 25 and 1,000 μm, preferably between 50 and 400 μm, morepreferably between 50 and 200 μm.

The protective cap should be broad enough to ensure that theinterconnect is indeed protected. It generally extends at least 10 μmbeyond the interconnect at both sides, preferably at least 50 μm.Generally, it will extend at most 200 μm beyond the interconnect at bothsides, preferably at most 100 μm. Obviously, if the protective cap isconductive, it may not extend beyond the groove interrupting the TCOlayer if that would mean that the two sides of the groove would becomeelectrically connected.

The grooves interrupting the front and back electrodes generally arecontinuous grooves, since the objective is to achieve proper insulationof the various cell strips. The interconnect, on the other hand, doesnot need to be continuous. If so desired, it may have a discontinuousform, taking the form of holes, stripes, or pits. For reasons ofoperational efficiency a continuous embodiment may be preferred.

The invention is illustrated by the following figures.

FIG. 1 shows process steps for making an interconnect between twoadjacent cells of a solar module of the invention.

FIG. 2 shows another embodiment the solar module.

FIG. 3 shows process steps for making an embodiment of the solar module.

FIG. 4 shows process steps for making an embodiment having anotherinterconnect in the solar module.

FIG. 5 shows an embodiment the solar module with insulating strips.

FIG. 6 shows another embodiment the solar module with insulating strips.

Various embodiments of the process according to the invention can beenvisaged.

A first embodiment is a variation on the series connection processdescribed in WO 98/13882. This process is illustrated in FIG. 1. Itcomprises the steps of:

-   -   providing an etchable temporary substrate    -   applying a front electrode of a transparent conductive oxide        (TCO) onto the temporary substrate and providing grooves in the        TCO layer down to the temporary substrate    -   applying a photovoltaic layer (PV layer) onto the TCO layer and        in the grooves present therein, and providing grooves in the PV        layer down to the TCO layer    -   applying a back electrode layer onto the PV layer and in the        grooves present in the PV layer so as to form an interconnect        which connects the back electrode layer to the TCO layer, and        providing grooves in the back electrode layer down to the PV        layer    -   applying a permanent carrier on the back electrode    -   in any one of the preceding steps applying an etch resist on the        non-TCO side of the temporary substrate at the location of the        interconnect    -   selectively removing the temporary substrate where it is not        covered with etch resist.

This process is illustrated in FIG. 1, wherein FIG. 1 a shows a solarcell foil comprising a temporary substrate 1 with on top of it a TCO 2a;2 b, a PV layer 3, a back electrode 4, and a permanent substrate 5.The interconnect 6 connects the TCO layer to the back electrode 4. Anetch resist cap 7 is provided on the temporary substrate at the locationof the interconnect. In FIG. 1 b the same system is shown after removingthe temporary substrate except where it is provided with the etch resistcap. No etch resistant is present at the location of the front groovefg, thus no remainder of the temporary substrate (usually a conductorsuch as aluminum) is present at the location of the front groove fg tocause short-circuit between separated TCO parts 2 a and 2 b.

A second embodiment, some steps of which are illustrated in FIG. 2, is aprocess comprising the steps of:

-   -   providing an etchable temporary substrate    -   applying a front electrode of a transparent conductive oxide        (TCO) onto the temporary substrate    -   applying a photovoltaic layer (PV layer) onto the TCO layer and        providing grooves in the PV layer down to the TCO layer    -   applying a back electrode layer onto the PV layer and in the        grooves present in the PV layer so as to form an interconnect        which connects the back electrode layer to the TCO layer, and        providing grooves in the back electrode layer down to the PV        layer or through the PV layer down to the TCO layer    -   applying a permanent carrier    -   in any one of the preceding steps providing an etch resist on        the non-TCO side of the temporary substrate at the location of        the interconnect    -   selectively removing the temporary substrate where it is not        covered with etch resist    -   providing a groove through the TCO layer down to the PV layer or        down to the back electrode from the side of the TCO layer where        the temporary substrate was present.

FIG. 2 a shows a solar cell foil comprising a temporary substrate 1 withon top of it a TCO 2, a PV layer 3, a back electrode 4, and a permanentsubstrate 5. The interconnect 6 connects the TCO layer to the backelectrode. An etch resist cap 7 is provided on the temporary substrateat the location of the interconnect. In

FIG. 2 b the same system is shown after removing the temporary substrateexcept where it is provided with the etch resist cap. Further, a groovefg has been established in the TCO layer so as to remove theshort-circuit and complete the series connection.

In a preferred version of this embodiment, protective strips of aninsulating material 8,9 are applied onto the PV layer before the backelectrode is applied. The strips are applied at the location where indue course the grooves in the back electrode and in the TCO layer willbe obtained. These strips are intended to protect the underlyingmaterial during the provision of the grooves, e.g., through laserscribing. This embodiment is illustrated in FIG. 2 c.

A third embodiment of the process according the invention comprises thefollowing steps:

-   -   providing an etchable temporary substrate    -   applying a front electrode of a transparent conductive oxide        (TCO) onto the temporary substrate    -   applying a photovoltaic layer (PV layer) onto the TCO layer and        providing grooves through the PV layer down to the TCO layer or        through the TCO layer down to the temporary substrate    -   applying a back electrode layer onto the PV layer and in the        grooves in such a way that inside the groove the back electrode        is interconnected to the front electrode, but the back        electrodes on both sides of the groove are electrically        insulated from each other    -   applying a permanent carrier    -   in any one of the preceding steps providing an etch resist on        the non-TCO side of the temporary substrate at the location of        the interconnect    -   selectively removing the temporary substrate where it is not        covered with etch resist    -   providing a groove through the TCO layer down to the PV layer or        down to the back electrode from the side of the TCO layer where        the temporary substrate was present.

Some steps of one embodiment of this process are illustrated in FIG. 3.FIG. 3 a shows a solar cell foil comprising a temporary substrate 1 withon top of it a TCO 2, a PV layer 3, a back electrode 4, and a permanentsubstrate 5. A groove g is present through the PV layer and through theTCO layer down to the temporary substrate. An etch resist cap 7 isprovided on the temporary substrate at the location of the groovethrough the PV layer and the TCO layer. In FIG. 3 b the same system isshown after removing the temporary substrate except where it is providedwith the etch resist cap. The remaining part of the temporary substratealso functions to connect the back electrode from one side of the grooveto the TCO layer at the other side of the cell. Further, a groove fg hasbeen established in the TCO layer so as to remove the short-circuit andcomplete the series connection. One of the features of this process isthat the protective cap also functions as part of the interconnect.Therefore, for this embodiment, the etchable temporary substrate shouldbe conductive.

A second version of this embodiment is illustrated in FIG. 4. In thisembodiment, the groove g in the PV layer does not extend through the TCOlayer. In this embodiment the protective cap does not function as partof the interconnect, so it does not need to be conductive. Neverthelessthe use of a conductive temporary substrate is preferred.

A fourth embodiment of the present invention comprises the followingsteps:

-   -   providing an etchable temporary substrate    -   applying a front electrode of a transparent conductive oxide        (TCO) onto the temporary substrate    -   applying a photovoltaic (PV) layer onto the TCO, and providing a        groove through the PV layer and the TCO down to the temporary        substrate, filling it with an insulating material, optionally        while applying a cap, and, if so desired, applying an insulating        strip, and establishing a (precursor of) a conductive connection        through the PV layer down to the TCO    -   applying a back electrode onto the PV layer    -   providing a groove in the back electrode down to the PV layer        or, if present, down to the insulating strip    -   optionally applying a permanent carrier    -   in any one of the preceding steps providing an etch resist on        the non-TCO side of the temporary substrate at the location of        the interconnect    -   selectively removing the temporary substrate where it is not        covered with etch resist.

This process is illustrated in FIG. 5. FIG. 5 a shows a solar cell foilcomprising a temporary substrate 1 with on top of it a TCO 2 and PVlayer 3. A groove fg is present through the PV layer and the TCO layerdown to the temporary substrate. This groove fg will be filled with aninsulating material 8. A further groove 6 has been provided through thePV layer down to the TCO. This groove 6 will form the basis for theinterconnect, and may thus be seen as an interconnect precursor. In FIG.5 b strips of insulating material 8 and 9 have been provided, one in thegroove through the PV layer and the TCO down to the temporary substrate,and one on the PV layer on the other side of the interconnect precursor6. In FIG. 5 c back electrode 4 has been added, with a groove bg on topof insulating strip 9. The interconnect is thus completed. Additionally,an etch resist cap 7 is provided on the temporary substrate at thelocation of the interconnect. In FIG. 5 d the same system is shown afterapplying a permanent carrier 5 and removing the temporary substrateexcept where it is provided with the etch resist cap.

In this embodiment the (precursor of) the interconnect can be a groovewhich on application of the back electrode is filled with back electrodematerial. This is the embodiment illustrated in FIG. 5. Alternatively,although such a process is less favored at the moment, the groove can befilled with a conductive material in a separate step. Finally, if the PVlayer is an amorphous Si layer, a conductive connection can also beestablished by recystallising the amorphous Si to form a conductivematerial. This last option is the least favored one.

A fifth embodiment of the present invention comprises the followingsteps:

-   -   a. providing a temporary substrate,    -   b. applying a transparent conductive oxide (TCO)    -   c. applying a photovoltaic (PV) layer on the TCO    -   d. if so desired, applying one or two insulating strips onto the        PV layer    -   e. applying a back electrode onto the PV layer and, if they are        present, onto the insulating strips    -   f. if so desired, repairing shunts in the back electrode if no        interconnect has been established as yet    -   g. providing a groove in the back electrode down to the PV        layer, or if it is present, down to an insulating strip    -   h. if so desired, providing a permanent carrier    -   i. in any one of the preceding steps providing an etch resist on        the non-TCO side of the temporary substrate at the location of        the interconnect    -   j. selectively removing the temporary substrate where it is not        covered with etch resist.    -   k. providing a groove from the side of the TCO through the TCO        and, optionally, through the PV layer down to the back        electrode, or if it is present, down to an insulating strip    -   l. if so desired, applying an encapsulant onto the TCO layer,        in which process a conductive connection through the PV layer is        established    -   between steps f. and h, or    -   between steps c. and e.

FIG. 6 illustrates various products resulting from different embodimentsof this process. These figures all show a show a solar cell foilcomprising a TCO 2, a PV layer 3, a back electrode 4, and a permanentsubstrate 5. The interconnect 6 connects the TCO layer to the backelectrode. An etch resist cap 7 is present on the remainder of thetemporary substrate 1 to protect the interconnect. The advantage of theembodiment wherein the conductive connection is established betweensteps f. and h. is that it is possible to effect shunt repair on theassemblage of TCO layer, PV layer, and back electrode before theconductive connection between the TCO layer and the back electrode isestablished.

The product of FIG. 6 a is obtained by a process comprising the stepsof:

-   -   a. providing a temporary substrate,    -   b. applying a transparent conductive oxide (TCO)    -   c. applying a photovoltaic (PV) layer onto the TCO    -   d. applying a back electrode onto the PV layer    -   e. if so desired, repairing shunts in the back electrode    -   f. providing a groove in the back electrode down to the PV        layer, and providing an interconnect between the back electrode        and the TCO layer    -   g. providing a permanent carrier    -   h. in any one of the preceding steps providing an etch resist on        the non-TCO side of the temporary substrate at the location of        the interconnect    -   i. selectively removing the temporary substrate where it is not        covered with etch resist.    -   j. providing a groove from the side of the TCO through the TCO        down to the PV layer.

The system of FIG. 6 b differs from that of FIG. 6 a in that twoprotective strips of an insulating material are applied onto the PVlayer before application of the back electrode. The groove in the backelectrode is made from the back electrode side down to the insulatingstrip. The groove fg from the TCO side is made through the TCO layer andthe PV layer down to the other insulating strip.

In the embodiments of FIGS. 6 a and 6 b, the conductive connectionbetween the TCO layer and the back electrode can be established invarious ways. As the back electrode is already present at the time theinterconnect 6 is established, the following options present themselves.If in this embodiment the PV layer is an amorphous Si layer, theconductive connection can be established by fusing the back electrodeand the amorphous Si layer using a laser. Alternatively, it is possiblein these embodiments to first scribe a groove from the back electrodeside of the solar cell unit and establish or improve the interconnect byfilling the groove with a conductive material. However, this option isless preferred.

The product of FIG. 6 c differs from that of FIG. 6 a in that theinterconnect interrupts the TCO layer instead of the back electrode ofthe solar cell foil. It can be obtained through a process comprising thefollowing steps:

-   -   a. providing a temporary substrate,    -   b. applying a transparent conductive oxide (TCO)    -   c. applying a photovoltaic (PV) layer on the TCO    -   d. providing an interconnect through the PV layer and optionally        through the TCO layer    -   e. applying a back electrode onto the PV layer    -   f. providing a groove in the back electrode down to the PV layer    -   g. providing a permanent carrier    -   h. in any one of the preceding steps providing an etch resist on        the non-TCO side of the temporary substrate at the location of        the interconnect    -   i. selectively removing the temporary substrate where it is not        covered with etch resist.    -   j. providing a groove from the side of the TCO through the TCO        down to the PV layer.

The system of FIG. 6 d differs from that of FIG. 6 c in that twoprotective strips of an insulating material are applied onto the PVlayer before application of the back electrode. The groove in the backelectrode is made from the back electrode side down to the insulatingstrip. The groove from the TCO side is made through the TCO layer andthe PV layer down to the other insulating strip. In this embodiment, theconductive connection between the TCO layer and the back electrode canbe established in various ways. If in this embodiment the PV layer is anamorphous Si layer, the conductive connection can be established byrecystallising the Si layer to form a conductive connection using alaser. Alternatively, and preferably, it is possible in this embodimentto first scribe a groove through the PV layer and the TCO layer andestablish the interconnect by filling the groove with a conductivematerial.

The Temporary Substrate

The temporary substrate has to satisfy a number of conditions. It has tobe etchable, as has been explained above. It has to be sufficientlyheat-resistant to be able to endure the conditions prevailing during themanufacture of the solar cell foil, more particularly during thedeposition of the TCO and the PV layer. It has to be strong enough to beable to carry the solar cell foil during its manufacture. It has to beeasy to remove from the TCO layer without damaging the latter. If it isintended to play a role in the interconnect, it has to be sufficientlyconductive. The person skilled in the art will be able to select asuitable temporary substrate within these guidelines.

The temporary substrate employed in the process according to theinvention preferably is a foil of a metal or a metal alloy. Theprincipal reasons for this are that such foils exhibit goodconductivity, generally are able to withstand high processingtemperatures, are slow to evaporate, and are comparatively easy toremove using known etching techniques. Another reason to choose a metalfoil, more particularly aluminium or copper, is that in the end thesolar cell foil has to be provided with edge electrodes which have toconnect the solar cell foil to an apparatus or the electricity grid.Remaining pieces of temporary substrate may be used to this end, as aresult of which there is no need for separate provision of the edgeelectrodes.

Suitable metals include steel, aluminium, copper, iron, nickel, silver,zinc, molybdenum, chromium, and alloys or multi-layers thereof. Foreconomic reasons among others it is preferred to employ Fe, Al, Cu, oralloys thereof. Given their performance (and taking into account thematter of cost) aluminium, iron, and copper are preferred most.

Suitable etchants and techniques for removing metals are known, andwhile they differ per metal, the skilled person will be able to selectthe appropriate ones. Preferred etchants include acids (both Lewis andBrønstedt acids). Thus in the case of copper it is preferred to useFeCl₃, nitric acid or sulphuric acid. Suitable etchants for aluminiumare, e.g., NaOH, KOH, and mixtures of phosphoric acid and nitric acid.

If copper, optionally prepared by way of electrodeposition, is used astemporary substrate, it is preferred to provide the copper, optionallyvia electrodeposition, with a non-reducing diffusion barrier layer,e.g., an anti-corrosion layer, more particularly zinc oxide. This isbecause copper may have the tendency to diffuse through the TCO layer inthe PV layer. It is also possible to select a TCO capable of preventingsuch diffusion, e.g., SnO₂ or ZnO. The anti-diffusion layers can beapplied by means of for instance electrodeposition, or via PhysicalVapor Deposition (PVD) or via Chemical Vapor Deposition (CVD). Theanti-diffusion layer generally is removed from the TCO together with thetemporary substrate.

For ease of removal, the temporary substrate preferably is as thin aspossible. Of course, its thickness has to be such that other layers canbe provided on it and it has to be able to hold these together, but thisgenerally does not require it to be more than 500 μm (0.5 mm) thick. Thethickness preferably is in the range of 1 to 200 μm (0.2 mm). Dependingon the modulus of elasticity, the minimum thickness for a large numberof materials will be 5 μm. Accordingly, a thickness of 5-150 μm, moreparticularly 10-100 μm, is preferred.

The TCO Layer

Examples of suitable transparent conductive oxides (TCOs) are indium tinoxide, zinc oxide, zinc oxide doped with aluminium, fluorine, gallium orboron, cadmium sulphide, cadmium oxide, tin oxide, and, most preferably,F-doped SnO₂. Said last-mentioned transparent electrode material ispreferred, because it can form a desired crystalline surface with acolumnar light scattering texture when it is applied at a temperatureabove 400° C., preferably in the range of 500 to 600° C., orafter-treated at said temperature. It is precisely in the case of thisTCO material that the use of a temporary substrate capable ofwithstanding such a high temperature is extremely attractive. Inaddition, the material is resistant to most etchants and has a betterresistance to chemicals than the much-used indium tin oxide. Also, it isfar less costly.

The TCO can be applied by means of methods known in the field, e.g., bymeans of Metal Organic Chemical Vapor Deposition (MOCVD), sputtering,Atmospheric Pressure Chemical Vapor Deposition (APCVD), PECVD, spraypyrolysis, evaporation (physical vapor deposition), electrodeposition,electroless plating, screen printing, sol-gel processes, etc. orcombinations of these processes. It is preferred to apply andafter-treat the TCO layer at a temperature above 250° C., preferablyabove 400° C., more preferably between 450 and 600° C., so that a TCOlayer of the desired composition, properties and/or texture can beobtained.

The Buffer Layer

If so desired, a buffer layer may be present between the TCO layer andthe photovoltaic layer. The buffer layer is intended to protect the TCOlayer from the conditions prevailing during the deposition of the PVlayer. The nature of the buffer layer will depend on the nature of thePV layer. Suitable buffer layers for the various PV layers are known inthe art. For cadmium telluride CdS, In(OH,S) and Zn(OH,S) may bementioned. If in the present specification mention is made of depositingthe PV layer on the TCO, a buffer layer may or may not be present onsaid TCO.

The Photovoltaic (PV) Layer

After application of the TCO layer the PV layer can be applied in anappropriate manner. It should be noted here that in the presentdescription the term “PV layer” or “photovoltaic layer” comprises theentire system of layers needed to absorb the light and convert it intoelectricity. Suitable layer configurations are known, as are the methodsfor applying them. For the common general knowledge in this fieldreference may be had to Yukinoro Kuwano, “Photovoltaic Cells,” Ullmann'sEncyclopedia, Vol. A20 (1992), 161 and “Solar Technology,” Ullmann'sEncyclopedia, Vol. A24 (1993), 369.

Various thin film semiconductor materials can be used in manufacturingthe PV layers. Examples are amorphous silicon (a-Si:H), microcrystallinesilicon, polycrystalline amorphous silicon carbide (a-SiC) and a-SiC:H,amorphous silicon-germanium (a-SiGe), and a-SiGe:H. In addition, the PVlayer in the solar cell foil according to the invention may comprise CIS(copper indium diselenide, CuInSe₂), cadmium telluride (CdTe), CIGSS(Cu(In,Ga)(Se,S)), Cu(In,Ga)Se₂, ZnSe/CIS, ZnO/CIS, and/orMo/CIS/CdS/ZnO, and dye sensitised solar cells.

The PV layer preferably is an amorphous silicon layer when the TCOcomprises a fluorine-doped tin oxide. In that case the PV layer willgenerally comprise a set, or a plurality of sets, of p-doped, intrinsic,and n-doped amorphous silicon layers, with the p-doped layers beingsituated on the side receiving the incident light.

In the a-Si—H embodiment the PV layer will at least comprise a p-dopedamorphous silicon layer (Si-p), an intrinsic amorphous silicon layer(Si-i), and an n-doped amorphous silicon layer (Si-n). It may be thatonto the first set of p-i-n layers a second and further p-i-n layers areapplied. Also, a plurality of repetitive p-i-n (“pinpinpin” or“pinpinpinpin”) layers can be applied consecutively. By stacking aplurality of p-i-n layers, the voltage per cell is raised and thestability of the system is enhanced. Light-induced degradation, theso-called Staebler-Wronski effect, is diminished. Furthermore, thespectral response can be optimized by choosing different band-gapmaterials in the various layers, mainly the i-layers, and particularlywithin the i-layers. The overall thickness of the PV layer, moreparticularly of all the a-Si layers together, will generally be of theorder of 100 to 2,000 nm, more typically about 200 to 600 nm, andpreferably about 300 to 500 nm.

The Back Electrode

The back electrode in the thin film solar cell sheet according to theinvention preferably serves both as reflector and as electrode.Generally, the back electrode will have a thickness of about 50 to 500nm, and it may comprise any suitable material having light reflectingproperties, preferably aluminium, silver, or a combination of layers ofboth, and making good ohmic contact with the subjacent semiconductorlayer. Preferably, it is possible to apply the metal layers at acomparatively low temperature, say less than 250° C., by means of, e.g.,electrodeposition, (in vacuo) physical vapor deposition or sputtering.In the case of silver, it is preferred to first apply an adhesionpromoter layer. TiO₂, TiN, ZnO, and chromium oxide are examples ofsuitable materials for an adhesion promoter layer and have the advantageof also possessing reflecting properties when applied in a suitablethickness, e.g., of 50-100 nm. The required back electrode may be eithertransparent or opaque.

The Permanent Carrier

The solar cell foil is provided with a permanent carrier. For, otherwisethe foil will be so thin that its fragility makes for difficulthandling. The permanent carrier is applied on the back electrode.Suitable carrier layer materials include films of commercially availablepolymers, such as polyethylene terephthalate, poly(ethylene2,6-naphthalene dicarboxylate), polycarbonate, polyvinyl chloride, PVDF,PVDC, PPS, PES, PEEK, PEI or films of polymer having very goodproperties such as aramid or polyimide films, but also, for example,metal foils onto which an insulating (dielectric) surface layer may havebeen applied, or compositions of plastics and reinforcing fibres andfillers. Polymeric “co-extruded” films provided with a thermoplasticadhesive layer having a softening point below that of the substrateitself are preferred. If so desired, the co-extruded film may beprovided with an anti-diffusion layer of, e.g., polyester (PET),copolyester or aluminium. Preferably, the carrier is a film or foil witha thickness of preferably 50 μm to 10 mm. Preferred ranges are 75 μm to3 mm and 100 μm to 300 μm. The bending stiffness of the carrier, definedwithin the context of this description as the product of the modulus ofelasticity E in N/mm² and the thickness t to the power of three in mm(E×t³), preferably is higher than 16×10⁻² Nmm and will generally belower than 15×10⁶ Nmm. The carrier is preferably suitable for use in aroll-to-roll process.

The carrier may comprise a structure as required for its final use. Thusthe substrate may comprise tiles, roofing sheets and elements, facadeelements, car and caravan roofs, etc. In general, however, preference isgiven to the carrier being flexible. In that case a roll of solar cellfoil is obtained which is ready for use and where sheets of the desiredpower and voltage can be cut off the roll. These can then beincorporated into (hybrid) roof elements or be applied onto tiles,roofing sheets, car and caravan roofs, etc., as desired.

If so desired, a top coat or surface layer may be provided on the TCOside of the solar cell to protect the TCO from outside influences.Generally, the surface layer will be a polymer sheet (with cavities ifso desired) or a polymer film. The surface layer is required to have ahigh transmission and for instance comprises the following materials:(per)fluorinated polymers, polycarbonate, poly(methylmethacrylate), PET,PEN or any clear coating available, such as the ones used in the carindustry. If so desired, an additional anti-reflection or anti-foulinglayer may be provided. Alternatively, if so desired, the entire solarcell may be incorporated into such an encapsulant.

The Etch Resist

The etch resist can be any material which can be applied to thetemporary substrate at the location of the interconnect which willprotect the temporary substrate from the action of the etchant. Theskilled person can select suitable material by routine testing. Suitableetch resists include thermoplastic and thermoset polyurethanes andpolyimides, thermoset polymers such as EP, UP, VE, SI, (epoxy)resins,and acrylates, and thermoplastic polymers such as PVC, PI,fluorpolymers, etc. The etch resist generally includes additives such asphotoinitiators or other hardeners, fillers, plastifiers, etc. The etchresist may be temporary, that is, it may be removed at some furtherstage of the process. Alternatively, and preferably, the etch resist maybe permanent.

The etch resist is suitably applied by vaporising or printing/writing.Preferably, the etch resist is applied by means of a printing processknown as such. Suitable printing processes include silk screening, rotoscreen printing, ink-jet processes, flexgravure, etc. The color of theetch resist can be regulated by the incorporation of suitable pigmentsor dyes known to the skilled person. Especially for permanent etchresists, the presence of pigments and UV stabilisers may be preferred.

1. A process for manufacturing a solar cell foil comprising the stepsof: providing an etchable temporary substrate applying a front electrodeof a transparent conductive oxide (TCO) onto the temporary substrateapplying a photovoltaic layer onto the TCO layer applying a backelectrode layer applying a permanent carrier ensuring that the frontelectrode and the back electrode are electrically connected in aninterconnect to establish a series connection, the front and the backelectrode each being interrupted by front and back groove, respectively,at different sides of the interconnect in any one of the preceding stepsproviding an etch resist on the non-TCO side of the temporary substrateat least at the location of the interconnect, and at least not at theentire location of the front groove selectively removing the temporarysubstrate where it is not covered with etch resist.
 2. The process ofclaim 1, wherein the step of applying the etch resist on the non-TCOside of the temporary substrate is performed directly before the step ofselectively removing the temporary substrate where it is not coveredwith etch resist.
 3. The process of claim 1, wherein the etch resist isa permanent etch resist.
 4. The process of claim 3, wherein the color ofthe etch resist is selected such that it matches or contrasts with thecolor of the energy-generating part of the solar cell unit.
 5. Theprocess of claim 1, wherein the etch resist is a temporary etch resist.6. The process of claim 1 which is carried out in a roll-to-rollprocess.
 7. A solar cell unit comprising a front electrode, a PV layer,and a back electrode layer, wherein the solar cell unit is divided intoat least two individual cells connected in series, the series connectioncomprising an interconnect which electrically connects the frontelectrode of one cell with the back electrode of an adjacent cell, whilethe front and the back electrode are each interrupted at different sidesof the interconnect, in which solar cell unit a protective cap ispresent on the front electrode at the location of the interconnect, withthe protective cap being of a different material than the interconnect.8. The solar cell unit of claim 7 which is a flexible solar cell foilsuitable for handling in a roll-to roll process.
 9. The process of claim2, wherein the etch resist is a permanent etch resist.
 10. The processof claim 2, wherein the etch resist is a temporary etch resist.
 11. Theprocess of claim 2 which is carried out in a roll-to-roll process. 12.The process of claim 3 which is carried out in a roll-to-roll process.13. The process of claim 4 which is carried out in a roll-to-rollprocess.
 14. The process of claim 5 which is carried out in aroll-to-roll process.